The present invention relates to temperature sensitive electrical switching devices and, more particularly, to a temperature sensitive device which is normally open but which closes an electrical circuit when the temperature of the device is raised to a predetermined temperature level and which device provides an air gap path through the device to permit arcing across the gap when a voltage of sufficient magnitude is applied to the device.
Thermally actuatable switch constructions are presently known in which an electric circuit is opened or closed in response to the switch being heated to a predetermined temperature level. In U.S. Pat. No. 3,875,546, issued Apr. 1, 1975, to Merrill, and U.S. Pat. No. 3,519,972, issued July 7, 1970, to Merrill, both assigned to the assignee of the present invention, temperature responsive electrical switches are disclosed in which a sliding electrical contact is held against a second electrical contact by a relatively stiff spring, which spring bears upon a normally solid, heat fusible, nonconductive pellet. When the pellet fusion temperature is reached, the pellet will melt and the stiff spring will no longer oppose the force of a somewhat weaker spring which then moves the sliding contact away from the second contact.
A normally open electrical switching device is disclosed in U.S. Pat. No. 3,189,508, issued Apr. 27, 1965, to Merrill and assigned to the assignee of the present invention. In the device disclosed therein, a sliding contact is moved into electrical contact with a stationary contact after fusion of a nonconductive heat fusible material.
It is also known to provide momentary overvoltage protection for various types of electrical machines by placing arc contacts in parallel electrically with the power terminals of the machine to be protected. When there is a momentary surge of line voltage, the excessive potential across the contacts will result in arcing and the machinery will therefore be protected. If the overvoltage condition should exist for a substantial period of time, the high current will blow the line fuses in the power supply line, permanently disconnecting the protected machine from the source of line power. If, on the other hand, the overvoltage condition exists only for a relatively short period of time, the arcing across the protective arc gap contacts may cease before sufficient current has passed through the line fuse to cause the fuse to blow. The protected machine will, therefore, resume its normal operations.
Heat sensitive, normally open electrical switches have in the past been placed in physical proximity to a protected electrical machine and connected electrically in parallel with the machine. If the machine should overheat during operation, the switch will close, thus causing a large current to flow through the power lines and resulting in the line fuse being blown. Previously, such thermal protectors have been separate from overvoltage protection devices, and added labor required for their connection into the circuits. Additionally the use of two discrete devices has increased the cost of the protection circuitry.
Thus, it is seen that there is a need for a combined normally open heat sensitive switching device and an overvoltage arc protector in which the air gap is accurately controlled and in which the construction of the device is simple and its operation extremely reliable.